Echo cancellation in two-way communications systems is well known and well developed in the art. More specifically, in regard to the concept of echos, full duplex transmission systems provide for full bandwidth transmission in both directions over a common medium, such as a telephone line. A hybrid is utilized to separate the transmit and receive signals from a two-wire connection. During the transmission and reception processes, the receive signal can be corrupted by the transmit signal. The corruption is oftentimes referred to as an "echo" in that the corruption is substantially similar to the transmit signal, although attenuated in amplitude and delayed in phase in comparison. Many systems have been developed in the art for cancelling, or removing the echo from the receive signal. As examples among many others, consider the echo cancellers described in U.S. Pat. No. 4,464,545 to Werner and also in U.S. Pat. No. 4,087,654 to Mueller.
The concept of crosstalk is similar to the concept of an echo and is at issue in the present application because crosstalk is a significant limiting performance factor in broadband data communication technologies, such as asymmetric digital subscriber line (ADSL). "Crosstalk" is defined as interference resulting from the inadvertent coupling of a signal on a first connection associated with a first communications channel onto a second connection associated with a second communications channel that is different than the first. The coupling usually occurs as a result of the close proximity of the wires of the respective connections. Each connection may carry a communications channel that is one directional or two directional, as well as half duplex or full duplex. The context in which crosstalk is generally a significant problem is in a transceiver bank that contains one or more transmitters and one or more receivers. In this configuration, a receiver in the transceiver bank may have a receiver that receives a corrupted receive signal that has been corrupted via crosstalk by one or more transmitters associated with other different communications channels. Echo corruption is different than crosstalk corruption because echo corruption involves corruption imposed upon a receive signal by a transmitter that is associated with the same communications channel as the receive signal.
As shown by way of example in FIG. 1, a system 9 includes a transceiver bank 11 having a transmitter 12 for generating a transmit signal along a transmission connection 13 and a receiver 15 for receiving a receive signal along a reception connection 17. The transmit signal is associated with a first communications channel, and the receive signal is associated with a second communications channel. Each of the connections 13, 17 may be a single wire or two wire configuration, and the connections 13, 17 are often bundled together, making crosstalk a more prominent possibility. Crosstalk involves the unintentional and undesirable coupling of the transmit signal from the connection 13 associated with the first communications channel (e.g. channel #1) to the receive signal on connection 17 associated with the second communications channel (e.g., channel #2). This coupling is due to capacitive and/or inductive coupling between the wires themselves and/or between the transmitter 12 and the receiver 15. Crosstalk appears as noise on the reception connection 17 and is thus a limiting factor in error free performance of information signals on the reception connection 17. Furthermore, crosstalk is especially a limiting factor if the transmission connection 13 is transmitting a signal at high amplitude, while the reception connection 17 is receiving a signal at low amplitude.
Crosstalk may extend beyond more than two communications channels. Indeed, in many central site cases, dozens or hundreds of wires may be bundled together so that the net crosstalk at a particular receiver may be due to many different transmission sources. Also, worth noting is that crosstalk can also be imposed upon a transmit signal by one or more reception sources.
If the interfering signals have nonoverlapping frequency bands, then it may be possible that a filter can be associated with the receiver to filter off the crosstalk in order to permit accurate reception. However, if the frequency bands overlap to any extent, such filtering cannot adequately be accomplished.
Thus, a heretofore unaddressed need exists in the industry for a crosstalk canceller system and method for cancelling crosstalk in a transceiver bank between separate communications channels having respective connections that carry corresponding signals having overlapping frequency bands.